The present invention covers the improvement of acoustic self-guidance systems for submarine vehicles.
The invention is applicable more especially to the guidance of torpedoes towards a target, which contain an active sonar, when a torpedo is moving in shallow water or near a reverberating surface. It makes it possible also to distinguish between true and false targets such as decoys.
In such a system, a target detection level is sought which is high enough with respect to reverberation from the surface and sea bottom, the reverberation level being proportional to the angular aperture of the acoustic beam in the horizontal plane (bearing). The front dimensions of torpedoes and of the acoustic transducers fixed on the torpedo head are of the order of 250 mm. Hence, if it is necessary to operate with a bearing aperture of less than 3.degree., operation must be at frequencies higher than 100 kHz, which will be called high frequencies, for which absorption in the marine medium is high. This reduces the range.
To correct this disadvantage, it is known that the sonic energy can be passed in metallic bars of length L situated in the vehicle's sides along an axis parallel to that of the vehicle's motion under conditions such that the energy be concentrated in a direction at an angle of .PSI. with respect to this axis.
The apparent dimension of the transmitting transducer being L Sin .PSI., beam widths of the order of 10.degree. at 10 kHz can be obtained. However, because of flow noises when the vehicle moves, the best position for the acoustic transducers is still the front surface of the vehicle.
A solution to reduce the reverberation noise can be found in U.S. Pat. No. 3,723,954, which consists in filtering the useful signal around the frequency shifted by the Doppler effect due to the torpedo to target relative speed.
The disadvantge is that attenuation of the reverberation depends on the target speed.
It is known that the directivity of a sonar receiving at low frequency (frequency less than 30 kHz) can be increased by producing a parametric effect of two directional beams transmitting simultaneously at high frequency. This effect is obtained thanks to the non-linear properties of sea-water. It is also known that multiple channels can be formed in a parametric sonar as is shown in the American report AD-AO 22 215 naval research distributed by the NTIS. Two high frequency beams correspond to each space channel on transmission, their difference in frequency varying from one channel to another which enables the low frequency overall reception channels to be filtered. The reception channels thus formed are called "frequential channels". This device has the disadvantage of requiring transmission on as many frequencies as there are channels plus one.